1. Field of the Invention
The present invention relates to a backlight device having a light source at the side of a side surface thereof, for illuminating a display unit which does not emit light by itself, such as a liquid crystal display.
2. Description of the Related Art
In recent years, along with the progress of hardware, the use of notebook type personal computers having the function of a desktop computer, a personal digital assistant (PDA), and the like has spread widely.
At present, a display unit of the aforementioned notebook type personal computer or the personal digital assistant generally employs a liquid crystal display panel.
However, the aforementioned liquid crystal display panel itself does not have a light-emitting function. Therefore, as a method for enabling the liquid crystal display panel to display information, such liquid crystal display panels employ either a backlight type method in which a light source is placed at the back side of a liquid crystal display panel or a reflection type method in which light from the front side of a liquid crystal display panel is reflected. Due to greater ability to see a display screen, the backlight type liquid crystal display panels are more widely used.
In Japanese Laid-open Publication No. 5-249461, for example, as a technique for the backlight structure, a reflection unit of the backlight is tapered so as to realize the miniaturization of the liquid crystal display panel.
FIG. 8 is a cross-sectional view showing a part of a liquid crystal display device having a conventional backlight device 102 and a liquid crystal display panel 101.
A diffusing sheet 109 is fixed on a surface f acing the liquid crystal display panel 101 of the light-guiding plate 111, and a white reflection sheet 108 is fixed on an opposite surface thereof. The diffusing sheet 109 and the white reflection sheet 108 are fixed at an edge of the light-guiding plate 111 with double-sided adhesive tapes 107. The light-guiding plate 111 has an output region 13 for outputting light incident from an end surface of the light-guiding plate 111 on the surface facing the liquid crystal display panel 101, and a plurality of prism sheets 110 are provided so as to include the output region 13 above the diffusing sheet 109.
In the case where the light-guiding plate 111 is made of an acrylic material, for example, essentially light rays each having a reflection angle of .theta.&gt;42.degree. with respect to the reflection sheet 108, which are input from an end surface 111a of the light-guiding plate 111, (e.g., light rays C and D in FIG. 8) are totally reflected at the interface between the light-guiding plate 111 and air layer 113. As a result, the light rays are not output outside through a surface 111b of the light-guiding plate 111. However, by printing a reflection pattern (not shown) on a surface 111c of the light-guiding plate 111 so as to change the reflective angle of such light rays to .theta.&lt;42.degree., light rays (e.g., light rays A and B in FIG. 8) are output from the surface 111b of the light-guiding plate 111.
According to the above-described conventional backlight device 102, however, in order to fix a reflector 106, the diffusing sheet 109, and the reflection sheet 108 to the light-guiding plate 111, it is necessary to fix edges thereof to the light-guiding plate 111 by the double-sided adhesive tapes 107. Due to the influence of the double-sided adhesive tapes 107, leakage of light in the vicinity of the lamp 105 which is a light source, or glare in a display image occurs.
Specifically, according to such a conventional structure, although light rays are reflected at an interface of the light-guiding plate 111 and the air layer 113, light rays are irregularly reflected at an interface of the light-guiding plate 111 and the double-sided adhesive tape 107. Therefore, as shown in FIG. 8, light rays C and D travel through the light-guiding plate 111 while being reflected. On the other hand, of the light rays reflected at the interface of the light-guiding plate 111 and the double-sided adhesive tape 107, light rays A and B are directly output outside from the light-guiding plate 111.
Due to the light which is irregularly reflected by the double-sided adhesive tape 107 and leaked directly outside from the light-guiding plate 111, light leakage at the surface facing the liquid crystal display panel 101 in the vicinity of the double-sided adhesive tape 107 or glare in a display image occurs.
In the vicinity of the double-sided adhesive tapes 107, the amount of light output from the surface 111b of the light-guiding plate 111 is increased due to the irregular reflection caused by the pieces of double-sided tape 107, thereby increasing the amount of light input to the prism sheets 110. As a result, the amount of light refracted in the prism sheets 110 to be output towards the liquid crystal display panel 101 is increased especially in the vicinity of the pieces of double-sided adhesive tape 107. Consequently, glare in a display image occurs.
If the pieces of double-sided adhesive tape 107 which are the cause of the irregular reflection are eliminated, however, attachment of the reflection sheet 108 and the diffusing sheet 109 to the light-guiding plate 111 becomes loose.